One form of process for producing dish-shaped moldings or hollow bodies of thermoplastic material involves producing a preform which is at least approximately of a tubular configuration by the extrusion of thermoplastic material to a given length, and then axially dividing the preform to constitute at least one single-layer web of material. The web of material then has its final shape imparted thereto in a molding tool, in a procedure involving the first heat, namely the heat which the web still has from the extrusion procedure.
There are various endeavours aimed at producing dish-shaped moldings which are then possibly welded together to form hollow bodies, from flat pre-fabricated articles. Processes which are widely used for that purpose are the process known as the thermoforming process or the deep-drawing process. Those processes are comparatively expensive in terms of energy as they operate in the procedure referred to as using the second heat, thus involving a heating step for re-heating the article when it has already cooled down. A process of the above-indicated kind in which the preform is put into its final shape in a molding tool can be found for example in EP 1 110 697 A2. A process of the same general kind is also to be found in WO 01/60592 A1.
Above-mentioned EP 1 110 697 A2 describes a process for producing a fuel tank which is shaped by extrusion blow molding, in which a tubular preform is firstly extruded. On issuing from the extrusion head, the preform is cut up along its length by means of severing tools and divided into two curtain-like single-layer webs of material. For that purpose disposed beneath the extrusion head is a device for guiding and spreading the halves of the tubular preform when it has been cut open lengthwise. That guide device which is arranged substantially in a saddle roof configuration is provided with respective guide rollers over which the webs of plastic material, while still in a plastic condition, slide down. Arranged beneath the extrusion head is the molding tool comprising two mold halves each provided with a mold cavity portion. In that process dividing the tubular preform along its length is used to make it possible to arrange internal fitments to be mounted in the fuel tank between the webs of material in such a way that the fitments do not touch the preform prior to the beginning of the blow molding procedure. After those fitments have been suitably placed the mold halves of the molding tool close, in which case the webs of material forming the preform are welded together at the edges of the mold and squeezed off. The preform is then expanded within the mold cavity by blowing air being introduced thereinto.
WO 01/60592 A1 also describes a process for producing a fuel tank consisting of half-casing portions which are welded together. The half-casing portions are produced by extrusion of a tubular preform which, after extrusion, is spread out to form a flat, single-layer web of material which is in turn transformed into the finishedx half-casing portion in a mold comprising a cavity and a ram member.
Both in the case of the process disclosed in EP 1 110 697 A2 and also in the case of the process of WO 01/60592 A1 it is comparatively complicated and expensive for the tubular preform when it has been cut open to be opened out to form a flat preform which is as planar as possible. In both cases suitable guide elements, rollers and transportation means are required for that purpose. After the preform has been spread out it has to be accommodated by the molding tool and separated by the molding tool or optionally by separately provided tools from the subsequent extrusion portion which follows it as it is extruded from the extrusion head. Suitable operating means are also required for that purpose.
The device for guiding and spreading out the preform is found to be an impediment in particular when the extrusion apparatus is brought into operation or started up. For that purpose it may firstly be necessary to free the guides of the hardened material. The extrusion head and the guide devices are generally arranged at a height of about 5 meters above the ground so that preparatory procedures for bringing the apparatus into operation, which generally have to be carried out by hand, are usually difficult to implement.